I suggest a much simpler and less expensive approach:
Use a dual 30amp 240v. breaker in the main box out to a 30a female twist lock connector in a utility box. From there, you can plug in a flex cable running to your 6 ckt mini breaker panel that feeds the critical ckts. When necessary, unplug the pig tail male twist lock connector, and plug it into the cable coming from your generator, which is outside, of course. There is no possibility of backfeed, and it is totally under your control. I am presently building such an arrangement into the house that I have under construction; the pigtail can go either to a 4kw backup generator (120/240v), or to a future similarly rated RE (solar/wind/battery) setup. Good luck! Van, K3CZ, PE